By far the largest use of synthetic polymers is as plastic. A major portion of it is used as throwaway containers and packing materials. Since plastics do not disintergrate by themselves, they are not biodegradable over a period of time. Non-biodegradability is due the carbon-carbon bonds of addition polymers which are inert to enzyme catalysed reaction. These polymers create pollution problem.
Biodegradable polymers are the polymers that can be broken into small segments by enzyme catalysed reactions using enzymes produced by microorganisms. In biodegradable polymers, bonds that can be broken by the enzymes are inserted into the polymers. Therefore, when they are buried as waste, enzymes present in the ground can degrade the polymer.
One method involves inserting hydrolysable ester group into the polymer. For example, when acetal (I) is added during the polymerzation of alkene, ester group is inserted into the polymeric chains.
Aliphatic polyesters are important class of biodegradable polymers. Some examples are described below:
(1) Poly - Hydroxybutyrate-CO-b-Hydroxyvalerate (PHBV)-
It is a copolymer of 3-hydroxybutanoic acid and 3 hydroxypentanoic acid, in which the monomer units are connected by ester linkages.
CH3-CH(OH)-CH2-COOH CH3-CH2-CH(OH)-CH2 -COOH → where R = CH3,C2 H5
The properties of PHBV vary according to the ratio of both the acids. 3-Hydroxybutanoic acid provides stiffness and 3-hydroxypentanoic acid imparts flexibility to the co-polymer. It is used in special packaging of orthopaedic devices and even in controlled drug release. When a drug is put in a capsule of PHBV, it is released only after the polymer is degraded. PHBV also undergoes bacterial degradation in the environment.
(2) POLY (GLYCOLIC ACID) AND POLY (LACTIC ACID)-
They constitute commercially successful biodegradable polymers such as sutures. Dextron was the first bioabsorbable suture made for biodegradable polyesters for post - operative stitches.
MOLECULAR MASS OF POLYMER:
Normally, a polymer contains chains of varying lengths and therefore, its molecular mass is always expressed as an average. In contrast, natural polymers such as protein contains chain of identical length and hence, have definite molecular mass.
The molecular mass of a polymer is expressed as:
(a) Number average molecular mass (Mn):
Where Ni is the number of molecules of molecular mass Mi
(b) Weight average molecular mass (Mw):
Methods such as light scattering and ultracentrifuge depend on the mass of individual molecules and yield weight average molecular masses. It is determined by employing methods which depend upon the number of molecules present in the polymer sample viz. colligative properties like osmotic pressure.
The ratio of the weight and number average molecular masses (Mw/Mn) is called Poly Dispersity Index (PDI). Some natural polymers, which are generally monodisperesed, the PDI is unity (i.e. Mw=Mn).
In synthetic polymers, which are always polydispersed, PDI > 1 because Mw is always higher than Mn.